Golf club shaft

ABSTRACT

A golf club shaft ( 10 ) having a plurality of flexural-rigidity-reduced regions (R 1  through R 3 ) each disposed adjacently to a side rearward from a corresponding local maximum-value point (P 1  through P 3 ) in flexural rigidity in a full length of the golf club shaft ( 10 ) from a tip ( 11 ) thereof where a head is mounted to a butt ( 12 ) thereof where a grip is mounted. Each of the flexural-rigidity-reduced regions (R 1  through R 3 ) has a flexural rigidity value lower than a flexural rigidity value (maximum value) (Yp 1  through Yp 3 ) of the corresponding local maximum-value point (P 1  through P 3 ) in flexural rigidity. Thereby the golf club shaft ( 10 ) is entirely flexible.

This nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No(s) . 2003-368158 filed in Japan on Oct. 28, 2003, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a golf club shaft and more particularly to a golf club shaft flexible to increase the head speed thereof.

2. Description of the Related Art

In recent years, with the increase of powerless elderly and female golfers, there is a demand for development of a golf club shaft capable of hitting a ball a long distance even with a small power. To comply with this demand, it is important to make the golf club shaft lightweight, provide the golf club head with a high restitution performance, and make the head speed high when a player swings.

The head speed of the golf club shaft can be increased by making it flexible. Thus the conventional golf club shaft is so constructed that the increase amount of the flexural rigidity value thereof increases from its tip at which the head is mounted toward its butt at which the grip is mounted. Although the golf club shaft having such a rigidity distribution is apt to flex at its tip, the entire golf club shaft does not flex favorably. Thus there is room for improvement of the head speed of the golf club shaft. Therefore proposals of increasing the head speed by improving the rigidity distribution of the golf club shaft have been hitherto made.

In the golf club shaft disclosed in Japanese Patent Application Laid-Open No. 2002-35184 (patent document 1), as shown in FIG. 9, the flexural rigidity distribution curve C3 has the maximum value in the central part Y of the golf club shaft and the maximum value, at the butt (rear end of golf club shaft) B, larger than the maximum value in the central part Y.

The golf club shaft disclosed in Japanese Patent Application Laid-Open No. 2003-24490 (patent document 2) has the following rigidity distribution: the central reinforcing layer in which the fiber of the reinforced fiber has an orientation angle of 0° to 5° is formed at the central part of the golf club shaft. As shown in FIG. 10, at the central part of the golf club shaft, the value (EI value) of the flexural rigidity thereof increases from its tip toward its butt. At the parts of the golf club shaft other than the central part thereof, the increase rate of the flexural rigidity becomes higher from its tip toward its butt, whereas at the central part of the golf club shaft, the increase rate of the flexural rigidity becomes lower from its tip toward its butt. Thereby the rigidity of the central part of the golf club shaft is higher than that of the central part of the conventional golf club shaft. Thus a player can utilize the flexing of the entire golf club shaft when the player swings, thus increasing the head speed.

However, the golf club shaft disclosed in the patent document 1 has only one portion having the maximum rigidity value other than the maximum rigidity value disposed at the butt side. Thus the golf club shaft flexes locally at the rigidity value-reduced portion disposed forward and rearward from the position having the maximum rigidity value. Therefore the player is liable to have a feeling of physical disorder. In addition a stress concentrates on the rigidity value-reduced portion disposed forward and rearward from the position having the local maximum rigidity value. Consequently the golf club shaft a low strength.

In the golf club shaft disclosed in the patent document 2, only the central part of the golf club shaft where the central reinforcing layer is provided is heavy and thick. Therefore the player has a feeling of physical disorder when the player swings. In addition difference in level is formed at the boundary between the central reinforcing layer and other parts. Thus the golf club shaft has an unfavorable accomplishment.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above-described problems. Therefore it is an object of the present invention to provide a golf club shaft which can be swung preferably without a feeling of physical disorder, flexes well entirely, and is high in its head speed.

To achieve the object, the golf club shaft of the present invention has a plurality of flexural-rigidity-reduced regions in an axial direction from a tip side of said golf shaft toward a butt side thereof, a local maximum-value point of the flexural-rigidity value is formed in each region interposed between two of said adjacent flexural-rigidity-reduced regions, so that a plurality of said local maximum-value points are disposed in the axial direction from a tip side of said golf shaft toward a butt side thereof.

The flexural rigidity value of the flexural-rigidity-reduced region interposed between the adjacent local maximum-value points is set to not more than that of the tip-side local maximum-value point in flexural rigidity adjacent to the flexural-rigidity-reduced region. When a local maximum-value point having a small flexural rigidity is present in one flexural-rigidity-reduced region, the local maximum-value point is included in the one flexural-rigidity-reduced region.

As described above, the flexural-rigidity-reduced regions each having a low flexural rigidity and making the shaft to flex easily are formed at a plurality of portions in the full length of the shaft. Therefore the shaft flexes entirely like a whip. Unlike the conventional shaft which flexes locally, namely, in only the vicinity of its tip or only at its central portion, the shaft of the present invention flexes entirely and greatly. Thus it is easy to increase the head speed and hence possible to hit a ball a long distance with a small power. Since the shaft flexes entirely, a player can swing smoothly without a feeling of physical disorder. Accordingly there is a small variation in the distance between a hitting point and the sweet spot. Thus it is possible to improve the directional property of a ball.

It is possible to realize a natural flexing of the shaft by providing the shaft with a large number of flexural-rigidity-reduced regions. It is preferable to form the flexural-rigidity-reduced region at least three portions for a wood. It is preferable to form the flexural-rigidity-reduced region at four portions for a long wood. It is preferable to form the flexural-rigidity-reduced region at least two portions for an iron. It is preferable to form the flexural-rigidity-reduced region at three portions for a long iron.

The flexural rigidity values of the local maximum-value points in flexural rigidity are equal to each other or increase in a direction from the tip side toward the butt side; and flexural rigidity values of local minimum-points of the flexural-rigidity-reduced regions are equal to each other or increase in the direction from the tip side toward the butt side.

If the flexural rigidity values of the local maximum-value points in flexural rigidity are equal to each other and if the flexural rigidity values of the local minimum-points of the flexural-rigidity-reduced regions are equal to each other, the shaft is capable of flexing entirely favorably without giving a feeling of physical disorder to the player.

If the flexural rigidity values of the local maximum-value points in flexural rigidity and if the flexural rigidity values of the local minimum-points of the flexural-rigidity-reduced regions increase in the direction from the tip side toward the butt side, the shaft gives player's hands a proper degree of firmness without having a feeling of physical disorder, when the player swings.

It is favorable that the length of each of the flexural-rigidity-reduced regions in the axial direction of the shaft is set to a range not less than 50 mm nor more than 400 mm. If the length of the flexural-rigidity-reduced region is larger than 400 mm, the number of the flexural-rigidity-reduced regions decreases. Consequently the degree of smooth flexing of the shaft becomes low and hence the player has a feeling of physical disorder when the player swings. If the length of the flexural-rigidity-reduced region is smaller than 50 mm, it is difficult to form the flexural-rigidity-reduced region. The length of the flexural-rigidity-reduced region in the axial direction of the shaft is more favorably not less than 50 mm nor more than 300 mm and most favorably not less than 50 mm nor more than 200 mm.

At least one of the flexural-rigidity-reduced regions is formed in a region from the tip to a position spaced by 500 mm from the tip. Thereby it is possible to reliably flex the shaft at its head-mounting side. Thus it is possible to increase the head speed to a higher extent.

It is preferable to form the local maximum-value point in flexural rigidity at both the tip and the butt. It is preferable that the flexural rigidity value is large to fix the head to the tip of the shaft. Thereby the butt side of the shaft is capable of giving the player a feeling that the shaft flexes smoothly.

It is preferable that a flexural rigidity value of the local maximum-value point in flexural rigidity at a tip side is set to a range of not less than 1.2 times nor more than two times as large as a flexural rigidity value of a local minimum-value point of the flexural-rigidity-reduced region adjacent to the local maximum-value point in flexural rigidity.

If the flexural rigidity value of the local maximum-value point in flexural rigidity at the tip side is set to less than 1.2 times as large as the flexural rigidity value of the local minimum-value point of the flexural-rigidity-reduced region, the shaft cannot be flexed sufficiently, and a low head speed-increasing effect is obtained. If the flexural rigidity value of the local maximum-value point in flexural rigidity at the tip side is set to more than 2.0 times as large as the flexural rigidity value of the local minimum-value point of the flexural-rigidity-reduced region, there is a big difference between the flexural rigidity value of the local maximum-value point in flexural rigidity at the tip side and the flexural rigidity value of the local minimum-value point of the flexural-rigidity-reduced region. Consequently the flexing degree of the shaft deteriorates.

The shaft is composed of a laminate of a plurality of prepregs each constructing a bias layer and a plurality of prepregs each constructing a straight layer. At least one layer of the straight layers serves as a flexural rigidity-adjusting layer having a plurality of separate prepregs, having an equal thickness and different elastic moduli, which are arranged in an axial direction of the shaft to form the flexural rigidity distribution. When the prepregs have different elastic moduli and an equal number of turns of the prepregs and an equal thickness to vary the flexural rigidity value of the shaft, the outer diameter of the shaft does not become locally large or small. Thus the shaft does not give the player a feeling of physical disorder in its external form, and further the weight of the shaft does not increase.

The above-described method is not limited to the formation of the shaft by layering prepregs one upon another. In addition, the above-described method can be used when filament winding is used to form the shaft. In this case, a reinforcing fiber whose elastic modulus is partly different can be used.

As apparent from the foregoing description, in the present invention, a plurality of flexural-rigidity-reduced regions whose values become lower from the tip side of the shaft toward the butt side thereof is formed over the full length of the shaft. Therefore the shaft is capable of flexing not locally but entirely smoothly like a whip and allows the player to make a natural swing without giving the player a feeling of physical disorder. Thereby the player can increase the head speed and hit a ball a long distance. Further it is possible to improve the directional property of a ball. At least one flexural-rigidity-reduced region is formed in the region from the tip to the position spaced by 500 mm from the tip. Thus it is easy to travel the golf club head easily and increase the head speed efficiently.

The flexural rigidity values of the local maximum and minimum points in flexural rigidity become increasingly large in the direction from the tip side of the shaft toward the butt side thereof. Consequently the shaft is capable of flexing naturally and smoothly, thus giving player's hands a proper degree of firmness. The flexural rigidity value of the tip-side local maximum-value point in flexural rigidity is set to the range of not less than 1.2 times nor more than two times as large as the flexural rigidity value of the local minimum-value point of the flexural-rigidity-reduced region adjacent to the tip-side local maximum-value point in flexural rigidity. Thereby the player obtains the effect of increasing the head speed and a feeling that the shaft flexes preferably.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a golf club shaft according to a first embodiment of the present invention.

FIG. 2 shows a layering construction of fiber reinforced prepregs, mounted on a mandrel, used for the golf club shaft according to the first embodiment of the present invention.

FIG. 3 is a graph showing a flexural rigidity distribution of the golf club shaft according to the first embodiment of the present invention.

FIG. 4 shows a layering construction of fiber reinforced prepregs, mounted on a mandrel, used for a golf club shaft according to a second embodiment of the present invention.

FIG. 5 is a graph showing a flexural rigidity distribution of the golf club. shaft according to the second embodiment of the present invention.

FIG. 6 shows a layering construction of fiber reinforced prepregs, mounted on a mandrel, used for a golf club shaft according to a third embodiment of the present invention.

FIG. 7 is a graph showing a flexural rigidity distribution of the golf club shaft according to the third embodiment of the present invention.

FIG. 8 is a graph showing flexural rigidity distribution curves A through F of examples 1 through 3 and comparison examples 1 through 3.

FIG. 9 is a graph showing a flexural rigidity distribution of a conventional art.

FIG. 10 is a graph showing a flexural rigidity distribution of another conventional art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the present invention will be described below with reference to drawings.

FIGS. 1 through 3 show a golf club shaft (hereinafter referred to as merely shaft) 10 according to a first embodiment of the present invention. The shaft 10 is tapered, long, and cylindrical. The shaft 10 is composed of a laminate of prepregs 21 through 24 layered one upon another. A head 13 is attached to the shaft 10 at one end (tip) 11 thereof having the smallest diameter. A grip 14 is attached to the shaft 10 at the other end (butt) 12 having the largest diameter. The entire length of the shaft 1 is set to 0.8 to 1.2 mm. In this embodiment, the entire length of the shaft 10 is set to 1050 mm. The outer diameter of the shaft 10 at its tip is set to 8.0 to 12.0 mm. In this embodiment, the outer diameter of the shaft 10 at its tip 11 is set to 9.0 mm. The outer diameter of the shaft 10 at its butt 12 is set to 13.0 to 20.0 mm. In this embodiment, the outer diameter of the shaft 10 at its butt is set to 15.0 mm.

The shaft 10 is formed as follows: After prepregs composed of resin and reinforcing fibers impregnated with the resin are wound around a mandrel 20 by a sheet winding method to layer them one upon another, a tape (not shown) made of polypropylene is lapped on the laminate of the prepregs. Thereafter the tape-lapped laminate is heated in an oven under pressure to harden the resin so that the resin and the reinforcing fibers are integrally molded. Thereafter the mandrel 20 is drawn out of the laminate. After the surface of the shaft 10 is polished, it is painted.

The prepreg constructing the shaft 10 consists of prepregs 21, 22 each forming a bias layer and prepregs 23, 24 each forming a straight layer.

In the prepregs 21 through 24, reinforcing fibers F21, F22, F24, F23 a and F23 b, and F24 consisting of carbon fiber are impregnated with epoxy resin.

The weight percentage of the resin of each of the prepregs 21 through 24 is set to not less than 20 nor more than 45. The thickness of each of the prepregs is set to not less than 0.01 mm nor more than 0.3 mm and preferably not less than 0.05 mm nor more than 0.15 mm. The elastic modulus of each of the prepregs is set to not less than 5 t/mm² nor more than 100 t/mm². The flexural strength of each of the prepregs in a direction 0° with respect to the reinforcing fiber is set to 100 kgf/mm² nor more than 200 kgf/mm². The number of grams of each of the prepregs per 1 m² is set to not less than 5 g nor more than 500 g. The number of grams of the carbon fiber per 1 m² is set to not less than 5 g nor more than 300 g.

Glass fiber or boron fiber may be used as the reinforcing fiber. Thermosetting resin other than epoxy resin may be used as the resin.

More specifically, the thickness of each of the prepregs 21, 22 is set to 0.1 mm. Each of the prepregs 21, 22 is wound over the full length of the shaft. The mandrel 20 is wound with three turns of each of the prepregs 21, 22 to obtain the winding width thereof. The reinforcing fibers F21 and F22 form angles of +45° and −45° respectively with respect to the axis of the shaft. The elastic modulus of each of the reinforcing fibers F21 and F22 is set to 30 t/mm². The prepregs 21 and 22 are wound around the mandrel 20 with the prepregs 21 and 22 layered one upon another.

The prepreg 23 is composed of four prepregs 23 a having a high elastic modulus (hereinafter referred to as high-elastic-modulus prepreg 23 a) and three prepregs 23 b having a low elastic modulus (hereinafter referred to as low-elastic-modulus prepreg 23 b), constructing a straight layer for adjusting the flexural rigidity of the shaft 10, which have an equal thickness of 0.1 mm. The mandrel 20 is wound with two turns of the high-elastic-modulus prepregs 23 a and the low-elastic-modulus prepregs 23 b to obtain the winding width of the prepreg 23. The high-elastic-modulus prepregs 23 a and-the low-elastic-modulus prepregs 23 b are alternately wound in a length of 150 mm around the mandrel 20 with the high-elastic-modulus prepregs 23 a and the low-elastic-modulus prepregs 23 b arranged in the axial direction of the shaft 10.

More specifically, the high-elastic-modulus prepregs 23 a and the low-elastic-modulus prepregs 23 b each having a length of 150 mm are alternately arranged from the tip 11 of the shaft 10 toward the butt in the order of the high-elastic-modulus prepreg 23 a, the low-elastic-modulus prepreg 23 b, the high-elastic-modulus prepreg 23 a, the low-elastic-modulus prepreg 23 b, the high-elastic-modulus prepreg 23 a, the low-elastic-modulus prepreg 23 b, and the high-elastic-modulus prepreg 23 a.

The elastic modulus of the high-elastic-modulus prepreg 23 a is set to 30 t/mm². The elastic modulus of the low-elastic-modulus prepreg 23 b is set to 5 t/mm². The reinforcing fiber F23 a of the prepreg 23 a and the reinforcing fiber F23 b of the prepreg 23 b have an orientation angle not less than 0° nor more than 10° with respect to the axis of the shaft 10. In the first embodiment, each of the reinforcing fiber F23 a and the reinforcing fiber F23 a has an orientation angle of 0° with respect to the axis of the shaft to form the prepreg 23 as a straight layer.

The prepreg 24 has a thickness of 0.1 mm. The mandrel 20 was wound with two turns of the high-elastic-modulus prepregs 23 a and the low-elastic-modulus prepregs 23 b to obtain the winding width of the prepreg 24. The reinforcing fiber F24 has an orientation angle not less than 0° nor more than 10° with respect to the axis of the shaft. In the first embodiment, the reinforcing fiber F24 has an orientation angle of 0° with respect to the axis of the shaft to form the prepreg 24 as a straight layer. The prepreg 24 has an elastic modulus of 25 t/mm².

As described above, the high-elastic-modulus prepreg 23 a and the low-elastic-modulus prepreg 23 b of the prepreg 23 constructing the flexural rigidity-adjusting straight layer are alternately arranged along the axis of the shaft 10 to construct a flexural rigidity distribution in which a first local maximum-value point P1 (hereinafter referred to as merely local maximum-value point) through a fourth local maximum-value point P4 are formed along the axis of the shaft 10 at a given interval provided between the first local maximum-value point P1 and the second local maximum-value point P2, between the second local maximum-value point P2 and the third local maximum-value point P3, and between the third local maximum-value point P3 and the fourth local maximum-value point P4, as shown in FIG. 3.

More specifically, the first local maximum-value point P1 is axially formed at a position proximate to the tip 11 of the shaft 10. The second local maximum-value point P2 is axially formed at a position spaced at an interval of 225 mm from the first local maximum-value point P1. The third local maximum-value point P3 is axially formed at a position spaced at the interval of 225 mm from the second local maximum-value point P2. The fourth local maximum-value point P4 is axially formed at the butt spaced at the interval of 225 mm from the third local maximum-value point P3.

The flexural rigidity values (local maximum value) Yp1 through Yp4 of the first local maximum-value point through the fourth local maximum-value point are set to 2 kgm², 4 kgm², 6 kgm², and 8 kgm² respectively. Thus the following relationship establishes among the flexural rigidity values (local maximum values) Yp1 through Yp4: Yp1<Yp2<Yp3<Yp4. Thus the local maximum value Yp2 is twice as large as the local maximum value Yp1. The local maximum value Yp3 is 1.5 times as large as the local maximum value Yp2. The local maximum value Yp4 is 1.3 times as large as the local maximum value Yp3.

A first flexural-rigidity-reduced region R1 is formed at the butt side with respect to the first local maximum-value point P1, namely, between the first local maximum-value point P1 and the second local maximum-value point P2. The flexural rigidity value of the first flexural-rigidity-reduced region R1 is set to not more than the flexural rigidity value Yp1 of the first local maximum-value point P1. That is, in the first flexural-rigidity-reduced region R1, the rigidity value thereof is not more than that of the adjacent tip-side first local maximum-value point P1.

Similarly a second flexural-rigidity-reduced region R2 is formed at the butt side with respect to the second local maximum-value point P2, namely, between the second local maximum-value point P2 and the third local maximum-value point P3. The flexural rigidity value of the second flexural-rigidity-reduced region R2 is set lower than the flexural rigidity value Yp2 of the second local maximum-value point P2. Similarly a third flexural-rigidity-reduced region R3 is formed at the butt side with respect to the third local maximum-value point P3, namely, between the third local maximum-value point P3 and the fourth local maximum-value point P4. The flexural rigidity value of the third flexural-rigidity-reduced region R3 is set lower than the flexural rigidity value Yp3 of the third local maximum-value point P3.

In local minimum-value points Q1 through Q3 in flexural rigidity of the flexural-rigidity-reduced regions R1 through R3, the flexural rigidity values (local minimum values) Yq1, Yq2, and Yq3 of the local minimum-value points Q1, Q2, and Q3 in flexural rigidity are set to 1 kgm², 3 kgm², 5 kgm² respectively. Thus the following relationship establishes among the flexural rigidity values (local minimum values) Yq1 through Yq3: Yq1<Yq2<Yq3. Therefore the local maximum value Yp1 is twice as large as the local minimum value Yq1. The local maximum value Yp2 is 1.3 times as large as the local minimum value Yq2. The local maximum value Yp3 is 1.2 times as large as the local minimum value Yq3.

In the above-described construction of the shaft 10, the three flexural-rigidity-reduced regions R1 through R3 are formed each in the length of 250 mm in the axial direction of the shaft 10. Thus it is possible to flex the shaft 10 entirely like a whip, because the shaft 10 flexes greatly in the three flexural-rigidity-reduced regions R1 through R3. Therefore unlike the conventional shaft which flexes locally greatly, a player can swing preferably without a feeling of physical disorder, increase the head speed, and hit a ball a long distance.

The entire flexural-rigidity-reduced region R1 is disposed from the tip 11 of the shaft 10 to a position within 400 mm of the tip 11. Therefore it is easy to travel the golf club head easily and increase the head speed efficiently.

The local maximum values and the local minimum values are so set that the flexural rigidity value of the local maximum-value point at a tip side is set to a range of not less than 1.2 times nor more than two times as large as the flexural rigidity value of the local minimum-value point of the flexural-rigidity-reduced region adjacent to the local maximum-value point. Therefore the shaft 10 is capable of flexing entirely sufficiently and smoothly and increasing the head speed without giving a player a feeling of physical disorder.

The high-elastic-modulus prepregs 23 a and the low-elastic-modulus prepregs 23 b having an equal thickness and composing the prepreg 23 constructing the flexural rigidity-adjusting straight layer are wound over the full length of the shaft 10. Thus no difference in level is generated on the surface of the shaft. Hence the shaft 10 does not give a feeling of physical disorder to the player.

FIGS. 4 and 5 show the second embodiment of the present invention. The construction of the shaft 10 of the second embodiment is similar to that of the shaft of the first embodiment except that of the prepregs 21 through 24 constructing the shaft 10, the construction of the prepreg 23 constructing the rigidity-adjusting straight layer are different from that of the first embodiment. Therefore the same parts of the shaft of the second embodiment as those of the first embodiment are denoted by the reference numerals of the first embodiment, and description of the same parts of the shaft of the second embodiment as those of the first embodiment is omitted herein.

As shown in FIG. 4, in the prepreg 23 constructing the rigidity-adjusting straight layer, the high-elastic-modulus prepregs 23 a having a length of 150 mm and the low-elastic-modulus prepregs 23 b having a length of 300 mm are arranged alternately from the tip 11 of the shaft 10 toward the butt thereof in the order of the high-elastic-modulus prepreg 23 a, the low-elastic-modulus prepreg 23 b, the high-elastic-modulus prepreg 23 a, and the low-elastic-modulus prepreg 23 b. In addition, the high-elastic-modulus prepreg 23 a is disposed in a remaining portion of the shaft 10 at its butt side. The elastic modulus of the high-elastic-modulus prepreg 23 a is set to 30 t/mm². The elastic modulus of the low-elastic-modulus prepreg 23 b is set to 5 t/mm².

As shown in FIG. 5, local maximum-value points P1 through P3 are formed on the shaft 10 having the above-described construction. The following relationship establishes among the flexural rigidity values (local maximum values) Yp1 through Yp3 of the local maximum-value points P1 through P3: Yp1<Yp2<Yp3.

The flexural-rigidity-reduced region R1 is formed at the butt side with respect to the local maximum-value point P1, namely, between the local maximum-value point P1 and the local maximum-value point P2. The flexural rigidity value of the flexural-rigidity-reduced region R1 is set lower than the flexural rigidity value Yp1 of the local maximum-value point P1. The flexural-rigidity-reduced region R2 is formed at the butt side with respect to the local maximum-value point P2, namely, between the local maximum-value point P2 and the local maximum-value point P3. The flexural rigidity value of the flexural-rigidity-reduced region R2 is set lower than the flexural rigidity value Yp2 of the local maximum-value point P2. In the flexural-rigidity-reduced regions R1 and R2, the following relationship establishes among the flexural rigidity values (local minimum values) Yq1 and Yq2 of the local minimum-value points Q1 and Q2 in flexural rigidity: Yq1<Yq2.

The shaft 10 having the above-described construction has the three local maximum-value points P1 through P3 and the two flexural-rigidity-reduced regions R1 and R2. However, the difference between the local maximum flexural rigidity value and the local minimum flexural rigidity value in the second embodiment is larger than the difference between the local maximum flexural rigidity value and the local minimum flexural rigidity value in the first embodiment. Thus the shaft 10 of the second embodiment can be flexed as smoothly and greatly as the shaft of the first embodiment.

FIGS. 6 and 7 show the third embodiment of the present invention. The construction of the shaft of the third embodiment is similar to that of the shaft of the second embodiment except that the construction of the prepreg 23 constructing the rigidity-adjusting straight layer is different from that of the second embodiment in that the prepreg 23 a disposed at the butt side of the shaft 10 of the second embodiment is replaced with a prepreg 23 d having an elastic modulus different from that of the prepreg 23 a. Therefore the same parts of the shaft of the third embodiment as those of the first embodiment are denoted by the reference numerals of the second embodiment, and description of the same parts of the shaft of the third embodiment as those of the second embodiment is omitted herein.

As shown in FIGS. 5 and 6, in the fiber reinforced prepreg 23, the following high-elastic-modulus prepregs and low-elastic-modulus prepregs are arranged from the tip 11 of the shaft 10 toward the butt thereof in the order of the high-elastic-modulus prepreg 23 a having a length of 150 mm, the low-elastic-modulus prepreg 23 b having a length of 300 mm, a high-elastic-modulus prepreg 23 c having a length of 150 mm, and the low-elastic-modulus prepreg 23 b. In addition a high-elastic-modulus prepreg 23 d is disposed in a remaining portion of the shaft 10 at its butt side. The elastic modulus of the high-elastic-modulus prepreg 23 c is set to 20 t/mm². The elastic modulus of the high-elastic-modulus prepreg 23 d is set to 80 t/mm². The elastic modulus of the low-elastic-modulus prepreg 23 b is set to 5 t/mm².

As shown in FIG. 7, three local maximum-value points P1 through P3 are formed on the shaft 10 having the above-described construction. The flexural-rigidity-reduced region R1 is formed at the butt side with respect to the local maximum-value point P1, namely, between the local maximum-value points P1 and P2. The flexural rigidity value of the flexural-rigidity-reduced region R1 is set lower than the flexural rigidity value Yp1 of the local maximum-value point P1. The flexural-rigidity-reduced region R2 is formed at the butt side with respect to the local maximum-value point P2, namely, between the local maximum-value point P2 and the local maximum-value point P3. The flexural rigidity value of the flexural-rigidity-reduced region R2 is set lower than the flexural rigidity value Yp2 of the local maximum-value point P2. In the flexural-rigidity-reduced regions R1 and R2, the following relationship establishes among the flexural rigidity values (local minimum values) Yq1 and Yq2 of the local minimum-value points Q1 and Q2 in flexural rigidity: Yq1<Yq2.

In the shaft 10 having the above-described construction, the difference between the local minimum flexural rigidity value Yq1 and the local maximum flexural rigidity value Yp1 in the third embodiment is set larger than the difference between the local minimum flexural rigidity value Yq1 and the local maximum flexural rigidity value Yp1 in the second embodiment. Further the difference between the local minimum flexural rigidity value Yq2 and the local maximum flexural rigidity value Yp2 in the third embodiment is set larger than the difference between the local minimum flexural rigidity value Yq2 and the local maximum flexural rigidity value Yp2 in the second embodiment. However, both the ratio of the local maximum flexural rigidity value Yp2 to the local minimum flexural rigidity value Yq1 and the ratio of the local maximum flexural rigidity value Yp3 to the local minimum flexural rigidity value Yq2 are set to not more than two. Therefore it is possible to increase the head speed and flex the shaft 10 preferably. Further because the prepreg 23 d having a very high elastic modulus is disposed at the butt side, the rigidity in the vicinity of the grip 14 is high. Thus the shaft 10 gives player's hands a proper degree of firmness when the player swings.

EXAMPLES

The shaft of each of the examples 1 through 3 and the comparison examples 1 through 3 was prepared by differentiating the elastic moduli of prepregs constructing the rigidity-adjusting straight layer from each other and differentiating the dispositions and constructions of prepregs from each other. The same head was mounted on each of the shafts to conduct a comparison test for comparing head speeds, the distance between a hitting point and the sweet spot, and a feeling testers had at a ball-hitting time. Table 3 shows evaluations of the test results.

A head mounted on the shaft was made of titanium. The head had a volume of 380 cc and a weight of 200 g. A rubber grip specially made was mounted on the shaft. The number of the golf club was W#1. TABLE 1 Distance from tip 0˜150 mm 150˜300 mm 300˜450 mm 450˜600 mm 600˜750 mm 750˜900 mm 900˜butt side Example 1 Mandrel 5 6.5 8 9.5 11 12.5 14 diameter Prepreg a c a c a c a Example 2 Mandrel 5 6.5 8 9.5 11 12.5 14 diameter Prepreg a c c a c c a Example 3 Mandrel 5 6.5 8 9.5 11 12.5 14 diameter Prepreg d c c d c c e Comparison Mandrel 5 6.5 8 9.5 11 12.5 14 example 1 diameter Prepreg a a d d d a a Comparison Mandrel 5 6.5 8 9.5 11 12.5 14 example 2 diameter Prepreg a a a e a a a Comparison Mandrel 5 6.5 8 9.5 11 12.5 14 example 3 diameter Prepreg a a a a a a a

TABLE 2 Elastic modulus Thickness Number of grams of PP/mm² [t/mm²] [mm] [g/m²] a 30 0.1 125 b 25 0.1 125 c 5 0.1 125 d 40 0.1 125 e 80 0.1 125

TABLE 3 Distance of hitting Feeling Head speed point from SS (SD) (maximum number [m/s] [mm] of marks is 10) Example 1 40.3 1.5 8.6 Example 2 40.5 1.7 9.1 Example 3 39.9 1.7 8.8 Comparison 37.8 3.7 6.8 example 1 Comparison 38.9 2.8 4.3 example 2 Comparison 38.5 3.0 7.0 example 3

The shaft of each of the examples 1 through 3 and the comparison examples 1 through 3 was formed by a sheet winding method. In each shaft, two prepregs (a) forming the bias layer were wound three times at the inner-layer side of the prepreg constructing the rigidity-adjusting straight layer in such a way that reinforcing fibers of the prepregs (a) intersected with each other. A prepreg (b) (8255S-12 produced by Toray Industries Inc.) constructing the straight layer was wound twice at the outer-layer side to integrate the prepregs (a) and (b) with each other. Table 2 shows the elastic modulus of each of the prepregs (a) and (b), the thickness of prepregs, and the g/m² of prepregs (PP). The length of each shaft was set to 1050 mm. The mandrels had the same configuration. Each mandrel was tapered from the butt toward the tip. Table 1 shows the outer diameter of the mandrel.

Example 1

The construction of the shaft of the example 1 was the same as that of the first embodiment shown in FIG. 2. More specifically, the shaft had prepregs constructing the rigidity-adjusting straight layer. High-elastic-modulus prepregs (a) (MR35DC-125S produced by Mitsubishi Rayon Inc.) having a length of 150 mm and low-elastic-modulus prepregs (c) (E1026D-12N produced by Nippon Graphite Fiber Inc.) having a length of 150 mm were arranged alternately from the tip of the shaft toward the butt thereof. The high-elastic-modulus prepreg (a) was disposed in a remaining portion of the shaft 10 at its butt side. The total number of the high-elastic-modulus prepregs (a) and the low-elastic-modulus prepregs (c) was seven. The elastic modulus of the high-elastic-modulus prepreg (a) was set to 30 t/mm². The elastic modulus of the low-elastic-modulus prepreg (c) was set to 5t/mm². As a result, the distribution of the flexural rigidity was the same as that of FIG. 3 (curve A of FIG. 8).

Example 2

The construction of the shaft of the example 2 was the same as that of the second embodiment shown in FIG. 4. As prepregs constructing the rigidity-adjusting straight layer, the high-elastic-modulus prepregs (a) having a length of 150 mm and the low-elastic-modulus prepregs (c) having a length of 30 mm were arranged alternately from the tip of the shaft toward the butt thereof in the order of the high-elastic-modulus prepreg (a) having a length of 150 mm, the low-elastic-modulus prepreg (c) having a length of 300 mm. In addition, the high-elastic-modulus prepreg (a) was disposed in a remaining portion of the shaft 10 at its butt side. The elastic modulus of the high-elastic-modulus prepreg (a) was set to 30 t/mm². The elastic modulus of the low-elastic-modulus prepreg (c) was set to 5 t/mm². As a result, the distribution of the flexural rigidity was the same as that of FIG. 5 (curve B of FIG. 8).

Example 3

The construction of the shaft of the example 3 was the same as that of the third embodiment. As prepregs constructing the rigidity-adjusting straight layer, the following prepregs were disposed from the tip of the shaft toward the butt thereof in the order of a high-elastic-modulus prepreg (d) (HRX350C-130C produced by Mitsubishi Rayon Inc.) having a length of 150 mm, a low-elastic-modulus prepreg (c) having a length of 300 mm, the high-elastic-modulus prepreg (d) having a length of 150 mm, and the low-elastic-modulus prepreg (c) having a length of 300 mm. In addition a super-high-elastic-modulus prepreg (e) (E8026C-12S produced by Nippon Graphite Fiber Inc.) was disposed in a remaining portion of the shaft 10 at its butt side. The elastic modulus of the high-elastic-modulus prepreg (d) is set to 40 t/mm². The elastic modulus of the high-elastic-modulus prepreg (d) was set to 40 t/mm². The elastic modulus of the low-elastic-modulus prepreg (c) was set to 5 t/mm². As a result, the distribution of the flexural rigidity was the same as that of FIG. 7 (curve C of FIG. 8).

Comparison Example 1

As prepregs constructing the rigidity-adjusting straight layer, the following high-elastic-modulus prepregs were arranged from the tip of the shaft toward the butt thereof in the order of the high-elastic-modulus prepreg (a) having a length of 300 mm and the high-elastic-modulus prepreg (d) having a length of 450 mm. In addition, the high-elastic-modulus prepreg (a) was disposed in a remaining portion of the shaft 10 at its butt side. The elastic modulus of the high-elastic-modulus prepreg (a) was set to 30 t/mm². The elastic modulus of the high-elastic-modulus prepreg (d) was set to 40 t/mm².

Comparison Example 2

As the prepreg constructing the rigidity-adjusting straight layer, the following prepregs were disposed from the tip of the shaft toward the butt thereof in the order of the high-elastic-modulus prepreg (a) having a length of 450 mm and the super-high-elastic-modulus prepreg e having a length of 150 mm. In addition, the high-elastic-modulus prepreg (a) was disposed in a remaining portion of the shaft 10 at its butt side. The elastic modulus of the high-elastic-modulus prepreg (a) was set to 30 t/mm². The elastic modulus of the super-high-elastic-modulus prepreg e was set to 80 t/mm².

Comparison Example 3

The entire rigidity-adjusting straight layer consisted of one high-elastic-modulus prepreg (a) having an elastic modulus of 30 t/mm².

Method of Measuring Head Speed

H/S was computed by measuring the position of the head at 1000 μs and 3000 μs immediately before an impact time.

Method of Measuring Distance Between Hitting Point and Sweet Spot

The distance between a hitting point of and the sweet spot was measured to measure variations of the distance.

Method of Evaluating Feeling at Ball-Hitting Time

Ten testers were requested to hit 10 balls with each shaft to make evaluations on a maximum of 10 points. The testers gave higher marks for shafts which gave the testers a better feeling. Table 3 shows the average of marks given by the testers. The testers had a handicap of nine in average. The 10 testers was 48 in average.

As shown with the curves A through C of FIG. 8, in the shafts of the examples 1 through 3 each having a plurality of the flexural-rigidity-reduced regions, the flexural rigidity value thereof increased from the tip thereof toward the butt thereof. As shown in FIG. 3, the head speed of each of the shafts of the examples 1 through 3 was high and there was a small variation in the distance between the hitting point and the sweet spot. This indicates that the shafts flexed favorably and thus the testers could swing without a feeling of physical disorder. This was proved by the fact that high marks were given in the evaluation of feeling.

The shaft of the comparison example 1 did not have the flexural-rigidity-reduced region whose flexural rigidity value decreases from the tip thereof toward its butt. The shaft had a flexural rigidity value which was comparatively high at the central portion thereof. The shaft of the comparison example 2 , the flexural rigidity value increased locally greatly at a position of the central portion thereof, and the flexural-rigidity-reduced region was present at only one portion on the side rearward from the local maximum-value point. The shaft of the comparison example 3 , the flexural rigidity value increased with the increase of the diameter thereof from its tip toward its butt, and the flexural rigidity distribution curve was almost linear with inclining gently upward toward the right.

The shafts of the comparison examples 1 through 3 had only one flexural-rigidity-reduced region or no flexural-rigidity-reduced region. Table 3 indicates that the head speed of these shafts was low and that there was a large variation in the distance between the hitting point and the sweet spot. Therefore the shafts did not flex smoothly and the testers had a feeling of physical disorder in their swings. This was proved by the fact that low marks were given in the evaluation of feeling. 

1. A golf club shaft having: a plurality of flexural-rigidity-reduced regions in an axial direction from a tip side of said golf shaft toward a butt side thereof, a local maximum-value point of the flexural-rigidity value is formed in each region interposed between two of said adjacent flexural-rigidity-reduced regions, so that a plurality of said local maximum-value points are disposed in the axial direction from a tip side of said golf shaft toward a butt side thereof.
 2. The golf club shaft according to claim 1, wherein said flexural rigidity values of said local maximum-value points are equal to each other or increase in a direction from said tip side toward said butt side; and flexural rigidity values of local minimum-points of said flexural-rigidity-reduced regions are equal to each other or increase in said direction from said tip side toward said butt side.
 3. The golf club shaft according to claim 1, wherein a length of each of said flexural-rigidity-reduced regions in said axial direction of said golf club shaft is set to a range not less than 50 mm nor more than 400 mm.
 4. The golf club shaft according to claim 2, wherein a length of each of said flexural-rigidity-reduced regions in said axial direction of said golf club shaft is set to a range not less than 50 mm nor more than 400 mm.
 5. The golf club shaft according to claim 1, wherein at least one of said flexural-rigidity-reduced regions is formed in a region from said tip to a position spaced by 500 mm from said tip.
 6. The golf club shaft according to claim 2, wherein at least one of said flexural-rigidity-reduced regions is formed in a region from said tip to a position spaced by 500 mm from said tip.
 7. The golf club shaft according to claim 3, wherein at least one of said flexural-rigidity-reduced regions is formed in a region from said tip to a position spaced by 500 mm from said tip.
 8. The golf club shaft according to claim 1, wherein a flexural rigidity value of said local maximum-value point at a tip side is set to a range of not less than 1.2 times nor more than 2 times as large as a flexural rigidity value of a local minimum-value point of said flexural-rigidity-reduced region adjacent to said local maximum-value point.
 9. The golf club shaft according to claim 2, wherein a flexural rigidity value of said local maximum-value point at a tip side is set to a range of not less than 1.2 times nor more than 2 times as large as a flexural rigidity value of a local minimum-value point of said flexural-rigidity-reduced region adjacent to said local maximum-value point.
 10. The golf club shaft according to claim 3, wherein a flexural rigidity value of said local maximum-value point at a tip side is set to a range of not less than 1.2 times nor more than 2 times as large as a flexural rigidity value of a local minimum-value point of said flexural-rigidity-reduced region adjacent to said local maximum-value point.
 11. The golf club shaft according to claim 5, wherein a flexural rigidity value of said local maximum-value point at a tip side is set to a range of not less than 1.2 times nor more than 2 times as large as a flexural rigidity value of a local minimum-value point of said flexural-rigidity-reduced region adjacent to said local maximum-value point.
 12. The golf club shaft according to claim 1, comprising a laminate of a plurality of prepregs each constructing a bias layer and a plurality of prepregs each constructing a straight layer, wherein at least one layer of said straight layers serves as a flexural rigidity-adjusting layer having a plurality of separate prepregs, having an equal thickness and different elastic moduli, which are arranged in an axial direction of said golf club shaft to form said flexural rigidity distribution.
 13. The golf club shaft according to claim 2, comprising a laminate of a plurality of prepregs each constructing a bias layer and a plurality of prepregs each constructing a straight layer, wherein at least one layer of said straight layers serves as a flexural rigidity-adjusting layer having a plurality of separate prepregs, having an equal thickness and different elastic moduli, which are arranged in an axial direction of said golf club shaft to form said flexural rigidity distribution.
 14. The golf club shaft according to claim 3, comprising a laminate of a plurality of prepregs each constructing a bias layer and a plurality of prepregs each constructing a straight layer, wherein at least one layer of said straight layers serves as a flexural rigidity-adjusting layer having a plurality of separate prepregs, having an equal thickness and different elastic moduli, which are arranged in an axial direction of said golf club shaft to form said flexural rigidity distribution.
 15. The golf club shaft according to claim 5, comprising a laminate of a plurality of prepregs each constructing a bias layer and a plurality of prepregs each constructing a straight layer, wherein at least one layer of said straight layers serves as a flexural rigidity-adjusting layer having a plurality of separate prepregs, having an equal thickness and different elastic moduli, which are arranged in an axial direction of said golf club shaft to form said flexural rigidity distribution. 